With the increasingly stringent automotive emission requirements there is a need to maximize conversion of the three main pollutant gases HC, CO, and NOx. Most present catalyst formulations utilize either Pt-Rh or Pd-Rh bi-metal precious metals in the catalyst formulation. The Pt and Pd both are oxidizing precious metals and are used to oxidize the hydrocarbons and carbon monoxide to water and carbon dioxide, while the Rh precious metal is utilized to reduce NOx to nitrogen and oxygen. In order to maximize conversion of the three pollutant gases in both gasoline and alternatively fueled engines there is a need to utilize the three precious metals (Pt, Pd, and Rh) together in a catalyst formulation. The three precious metals would act synergistically to achieve maximum conversion efficiency. With the cost of the oxidizing precious metal Pt being 3 to 5 times the cost of Pd it would be cost effective to be able to reduce Pt content with increased Pd usage while at the same time improving emissions performance.
A problem associated with the usage of the three precious metals on one monolithic catalyst brick is that at the high temperatures encountered in automotive catalytic converters there is a sintering of the three precious metals. Sintering of the precious metals is defined as the coalescing of small precious metal clusters into larger precious metal clusters with the resulting effect of a loss in catalytic efficiency. There is a strong desire in automotive catalysis to reduce sintering. In the case of the trimetal precious metal system the sintering that occurs results in a surface enrichment of Pd over Pt and Rh. This Pd enrichment is undesirable and is due to the lower surface energy and lower heat of atomization of Pd compared to Pt and Rh. This sintering phenomena results in the Pt and Rh being buried under the Pd and thus not being able to contact the pollutant gases. In effect the Pt/Pd/Rh catalyst results in a Pd only catalyst. This sintering phenomena would especially adversely effect the Rh which is required for NOx conversion. In addition there would be an economic loss in that the reduced catalytic benefit would not justify the cost for the Pt and Rh precious metals.
The present invention provides advantages over the prior art.